Placed in sunlight or under a fluorescent lamp, especially at ultraviolet wavelengths, titanium oxide shows strong oxidation on its surface due to photocatalytic action. It is well-known that the oxidation removes organic compounds and toxic matter, such as bacteria, that are in contact with the titanium oxide. See Japanese patent 2883761 (published on Jun. 22, 1993; registered on Feb. 5, 1999; hereinafter, “patent document” 1), for example. However, titanium oxide is powdery; if it is used as such, the powdery titanium oxide (titanium oxide powder) may scatter in gas or fluid. Titanium oxide powder needs be in a solid form for convenient use. How this “fixing” of titanium oxide powder is done greatly affects the probability of the titanium oxide coming in contact with toxic substances. Further, if no light reaches titanium oxide, no photocatalytic action occurs. Conventionally, titanium oxide is fixed onto substrates of various shapes as described in patent document 1: many other alternatives are also proposed including films, glass beads, silica gel, and stainless steel wool.
In Japanese Unexamined Patent Publication, or Tokukai, 2001-179246 (published on Jul. 3, 2001; hereinafter, “patent document” 2), one of the inventors of the present invention suggested a purifier device using fine hollow glass spheres coated with titanium oxide. The purifier device (not shown) has a light source and an enclosure tube. The space between the light source and tube is filled with numerous fine, hollow glass spheres coated with titanium oxide. The device is capable of purifying polluted fluid by simple operation and without concern for secondary pollution. However, the fine, hollow glass spheres do not allow sufficient ultraviolet to reach the titanium oxide. Demands are there for further improvement on the photocatalytic action.
Hence, the purifier device of patent document 2 suggested by one of the inventors of the present invention is indeed innovative, but falls short of stretching the photocatalyst to the limit.
With growing use of photocatalysts, expectations are rising also for visible-light-responsive three-dimensional fine cell-structured photocatalytic filters that function in sunlight and under other visible light sources, as well as across the ultraviolet range of spectrum, at high efficiency. We expect that if we can purify polluted air and water in sunlight at high efficiency, we will not have to use electricity, and the use of photocatalysts will dramatically expand.